It is known that primary aromatic amines and their derivatives can be converted to arylisocyanide dichlorides in good yield by a number of methods, as for example the reactions graphically illustrated by the following equations taught by: E. Kuhle, B. Anders, and G. Zumach, Angewandte Chemie (International Edition), 6, 649 (1967); and J. E. Baldwin and J. E. Patriok, Chemical Communications, 968 (1968). ##EQU1## In these equations the legend "Ar" means aryl.
Although such reactions are effective for preparing arylisocyanide dichlorides from primary aromatic amines and derivatives thereof, they do not suggest a means for selectively or exclusively chlorinating phenylisocyanide dichlorides in the para-position. In fact to the contrary, the art reporting the above reactions actually indicates that these reactions yield (a) isomeric mixtures of haloarylisocyanide dichlorides, or (b) fail to achieve ring chlorination at all.
J. V. Nef, Ann. (Justus Liebigs), 270, 267 (1892), reported that chlorination of phenyl isothiocyanate at 0.degree.C. in chloroform solution yielded a mixture of phenylcarbonimidoyl dichloride and the p-chloro derivative thereof. Bly, Perkins and Lewis, J. Am. Chem. Soc., 44, 2896 (1922), attempted chlorination of phenylisothiocyanate in carbon tetrachloride and in phenylcarbonimidoyl dichloride but found no evidence of ring chlorination in either solvent system. D. B. Murphy, J. Org. Chem., 29, 1613, (1964), reported that when chlorination of phenylisothiocyanate is carried out in chloroform using excess chlorine a mixture of p-chlorophenyl carbonimidoyl dichloride, the unsubstituted compound and 2,4-dichlorophenyl carbonimidoyl dichloride, is obtained. Murphy also reported that when excess chlorine was removed from the reaction mixture immediately after the initial chlorination was completed, no ring chlorination occurred. In addition, we have found that chlorination of o-formotoluidide with a chlorine source such as sulfuryl chloride or chlorine gas leads to a mixture of approximately 75% 4-chloro- and 25% 6-chloro-o-formotoluidide. Similar results are also obtained with the acetyl derivative. Since these procedures give isomer mixtures, the additional undesirable step of isomer separation is required.
It was therefore an object of the present invention to find a process which would provide a means for chlorination of o-alkylaniline derivatives exclusively in the para-position.